A photomask or reticle may be used to transfer a pattern into a photoresist coated on a substrate. The reticle may have a profile or shape in cross-section that is affected by a method of fabrication or use. The reticle may be non-planar with a simple shape, such as a global concave or global convex shape. When a reticle frontside (which includes the pattern) has the global convex shape, the reticle backside may have a corresponding global concave shape. Alternatively, the reticle may be non-planar with a complex shape that mixes or combines multiple modes at different locations across the reticle, such as a local concave, local convex, or irregular shape.
The non-planar shape will cause gaps between portions of the reticle backside and portions of a frontside of a chuck which holds the reticle. Accordingly, a large force may be needed to sufficiently reduce the gaps to adequately conform the reticle backside to the chuck frontside. Consequently, sliding between the reticle backside and the chuck frontside may result in scratching damage and particle generation that can, in turn, lead to lithography errors. These problems are especially problematic for extreme ultraviolet (EUV) lithography, where stringent requirements for high flatness and minimal particle generation exist. Due to a requirement for projection of EUV light in a vacuum environment, an electrostatic chuck (e-chuck) may be used to hold the reticle in a scanner during exposure of the substrate such as a wafer.